Smart fortwo 0.8 CDI W451

The KP31 turbocharger uses a vacuum-operated wastegate that is prone to failure. The vacuum hoses feeding the wastegate actuator degrade and split over time, preventing proper boost control. The wastegate actuator itself can also seize or the internal gasket can leak, causing the wastegate to stick partially open. Symptoms include sudden power loss, increased fuel consumption, and limp mode without a check engine light. Repair costs range from around €250 for a vacuum hose and solenoid replacement to €800-950 for a full wastegate actuator replacement and recalibration. The turbo itself is generally robust, with many examples running well past 150,000 km on the original unit.

The automated manual (softip/softouch) gearbox relies on a clutch actuator with internal plastic gears that wear over time. Typical failure occurs between 80,000 and 130,000 km. Symptoms include jerky gear changes, inability to select reverse, delayed engagement, or the car refusing to move. Wiring between the actuator and the intercooler cradle is also prone to chafing. A replacement actuator from Smart/Mercedes costs around €500-600 for the part. Specialist repair of the internal gears is possible for €250-400. Full gearbox rebuild including clutch and actuator can reach €1,200. Updating the gearbox controller software to a later version can delay failure by raising the clutch engagement threshold.

This issue specifically affects the 54 hp CDI models (October 2009 onwards) that use an EGR cooler to meet Euro 5 emissions. The cooler units are poorly manufactured and develop internal cracks over time. When the cooler splits, engine coolant leaks directly into the exhaust system, producing visible steam from the tailpipe. If not caught early, continued coolant loss leads to engine overheating and potential head gasket damage. The first symptom is often that the cabin heater stops producing warm air (due to low coolant), followed by the temperature gauge rising. The EGR cooler is buried behind multiple engine ancillaries, making access extremely labor-intensive. The 45 hp CDI models do not have this component and are unaffected.

The 54 hp CDI (from October 2009) was equipped with a DPF to meet Euro 5 standards. The small 0.8-litre engine struggles to reach the exhaust temperatures needed for passive DPF regeneration, especially during short urban journeys. A blocked DPF triggers limp mode and the DPF warning light. Active regeneration requires sustained driving at over 80 km/h for at least 10-15 minutes during 3 out of 5 journeys. Professional DPF cleaning costs around €300-500, while a replacement DPF (aftermarket) costs €200-400 for the part plus €200-300 labor. OEM replacement from Smart/Mercedes can reach €1,500-1,800 installed. The DPF pressure sensor is the most common cause of false DPF warnings and safe mode, and costs around €80-150 to replace. The 45 hp CDI models (pre-October 2009) have no DPF.

The OM660 uses a timing chain rather than a belt. Over time, the chain stretches and the slide rails wear, eventually exceeding the tensioner's travel. Incorrect oil viscosity, poor-quality oil, irregular servicing, or a seized chain tensioner all accelerate wear. The first symptom is a ticking noise from the engine that progresses to a rattle. If ignored, the chain can grind through the soft aluminium timing cover (causing oil leaks) and eventually skip a tooth, leading to valve-to-piston contact. This issue typically appears above 150,000 km with good maintenance, or earlier if servicing has been neglected. Replacement involves the chain, tensioner, guides, and sprockets.

The EGR valve accumulates soot from exhaust gases, especially on cars driven mostly in urban traffic. A stuck-open valve causes rough idle and poor performance; a stuck-closed valve triggers fault code P2355 and limp mode. On the 54 hp model, the EGR solenoid vacuum valve wiring can also chafe against brake lines due to vibration, triggering fault code P2359. Cleaning the EGR valve is sometimes sufficient (€100-200 including labor), but if the solenoid has burned out or the valve body is corroded from coolant ingress (linked to the EGR cooler issue on 54 hp models), full replacement costs €400-600.

The SAM (Signal Acquisition Module) is the central electronic control unit managing all the car's electrical systems. It is not simply a fuse box. Water ingress through poorly sealed areas or condensation causes partial failures: intermittent hazard lights, non-functioning windows, central locking glitches, or fuel pump relay issues. A new SAM unit from Mercedes with coding costs €600-800. Specialist refurbishment and cloning services are available for €200-400, which involves repairing damaged circuits and reprogramming the unit. Early intervention when symptoms first appear can save significant cost.

Despite the plastic body panels that make Smarts appear rust-free, the underlying steel tridion safety cell and subframe can develop significant rust. The rear wheel arch area, hidden behind the arch liner, is the most common location. Salt and moisture accumulate between the plastic panels and steel structure. The steel inserts where the rear wing bolts to the door sill can rust, causing the insert to expand and crack the surrounding structure. Rust is the primary threat to long-term survival of these cars. Professional repair per arch costs €300-500, and severe structural rust can make the car uneconomical to repair.